In the first stage of this project we will study experimentally the factors affecting peripheral venous capacity, and will develop a model of its control as a function of body temperatures and exercise intensity. Such a model will contribute to a more complete understanding of the mechanisms involved in heat syncope and other heat disorders, in which venous pooling plays a prominent etiological role. We will measure esophageal temperature (Tes), skin temperatures, and forearm venous volume of subjects exercising on a bicycle ergometer at various combinations of ambient temperature and exercise intensity. Forearm volume will be measured with electrocapacitance plethysmography, and venous volume will be taken as the difference between the volume measured with veins filled to a standard congesting pressure, and volume measured with the veins emptied by external counter-pressure on the forearm. Having derived from these measurements a model predicting forearm venous volume in terms of Tes, mean skin temperature, and exercise intensity, we will then investigate how the control parameters of the basic model are affected by a program of exercise training and heat acclimation. We will also investigate the role of the veins in core to skin heat transfer by determining the relationship between thermal conductance and skin blood flow at several different levels of venomotor tone. In addition, we will determine the effect of increased peripheral venous volume on stroke volume, which limits cardiac output under conditions of heat strain. Additional work in this project includes investigating the effects of such factors as age, sex, and season on the control parameters, which will aid prediction of individual tolerance to heat stress.